1. Field of the Invention
The present invention relates to a heat-sensitive recording material and more specifically to a heat-sensitive recording material whose recording layer and hence the resulting recorded images are substantially improved in durability through the use of an emulsion of a specific copolymer in the intermediate layer, the recording layer or the protective layer thereof.
2. Description of the Prior Art
Presently, there have been widely and quickly applied a heat-sensitive recording material, in particular heat-sensitive recording paper, which comprises a substrate provided thereon with a heat-sensitive recording layer comprising a usually colorless or lightly colored chromophoric substance which is an electron donative substance, a phenolic compound or an electron acceptor which allows for the chromophoric substance to cause color development by heat and a binder as output recording paper for a variety of printers inclusive of facsimile; a printer for industrial instrumentation and for medical use; a POS system and a note issuing system.
However, various problems arise as the application thereof increases. For instance, the durability (such as water resistance, resistance to plasticizers and resistance to solvents) of recording layers and hence that of recorded images obtained after color development under the ordinary handling environment as will be detailed below are still insufficient. More specifically, there have been observed a variety of drawbacks such as peeling off of the recording layer when the heat-sensitive recording material is brought into contact with water, color development upon putting it on diazo light-sensitive paper immediately after copying, fading or discoloration due to plasticizers included in polyvinyl chloride upon putting it on a polyvinyl chloride mat or film and color development by the action of oils or solvents. These problems cannot be completely solved by the conventional techniques.
In general, there have been used, for instance, a known water-soluble polymer such as polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, casein and polyvinyl pyrrolidone as binders for a heat-sensitive recording layer and an intermediate layer to be arranged between a substrate and the heat-sensitive recording layer. These binders of course have low water resistance and, for this reason, recorded color developable images are peeled off when they are brought into contact with water. Therefore, the most important object is to impart water resistance to heat-sensitive recording materials since the frequency of coming in contact with water is the highest in the usual circumstances and thus many attempts have been directed to this object. For instance, Japanese Patent Unexamined Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 55-95593 discloses a method for enhancing the water resistance of a heat-sensitive recording material in which a combination of a polyvinyl alcohol and a butadiene-acrylate-styrene copolymer is used as a binder to cause self-crosslinking between them; J. P. KOKAI No. Sho 57-19036 discloses a method in which an isocyanate compound is added to a combination of a polyvinyl alcohol, an oxidized starch and an etherified starch serving as a binder; and J. P. KOKAI No. Sho 62-42884 discloses a method in which a combination of a polyvinyl alcohol, a latex and a crosslinking agent is employed as a binder.
However, in these methods wherein self-crosslinking is utilized or a crosslinking agent is added, the properties of a coating color for forming a heat-sensitive recording material vary with time, and heat required for the crosslinking agent to form sufficient crosslinks cannot be applied to heat-sensitive recording paper during the production thereof. For these reasons, a catalyst capable of causing crosslinking at ordinary temperature must be added and then the recording paper must be aged over a long time period. Moreover, these binders mainly comprise a water-soluble polymer and, therefore, if such a resin is used at a high concentration, the resulting coating color correspondingly has a high viscosity. This creates a variety of problems concerning process control. For instance, the workability of coating operations becomes low and further, an aqueous dispersion of a resin having substantially low heat resistance is inevitably used for improving the water resistance of the resulting heat-sensitive material and thus the heat resistance thereof is liable to be impaired. Thus, the efficiency of the process is greatly impaired.
As has been explained above, the methods for improving binders per se suffer from a variety of problems from the viewpoint of operations and the improvement in the water resistance and the resistance to plasticizers of the resulting recording material are also insufficient. For this reason, in fields in which recording materials having higher durability are required, there have been adopted methods in which a protective layer is applied onto a heat-sensitive layer and an improved binder is used. For instance, J. P. KOKAI No. Sho 61-284483 discloses a method in which an aqueous ultraviolet ray- or electron beam-curable resin system is incorporated into a binder and/or a protective layer; J. P. KOKAI No. Sho 60-59193 discloses a method in which a protective layer is formed from a composition containing a water-soluble resin and a crosslinking agent.
The method in which a UV ray- or electron beam-curable resin is used has advantages such as quick curing without heating, but they have only limited applications and these heat-sensitive materials are inferior to those obtained according to conventional ones from the viewpoint of cost, since the production line must be renewed or newly established and materials are expensive. In the method in which a protective layer comprising a water-soluble polymer and a crosslinking agent is used, the viscosity of a coating color of a resin becomes very high when the resin is used in a high concentration and correspondingly the workability is also lowered.
Moreover, since a crosslinking agent is an essential component of the coating color, problems concerning process control arise. For instance, the properties of the coating color vary with time, heat sufficient for ensuring a crosslinking reaction of the crosslinking agent cannot be applied to the coating color during the production of heat-sensitive layer and hence the coated layer must be sufficiently aged over a long time period.
In other words, the foregoing methods make it possible to solve the aforementioned problems to some extent, but these methods are not satisfactory in view of quality of the resulting heat-sensitive material inclusive of resistance to heat softening (resistance to sticking) of a protective layer optionally applied and from the viewpoint of process control.